1. Field
The present disclosure relates generally to light emitting diodes (LED), and more particularly, to a method and apparatus for drain switching with replication loop to achieve fast turn on time of an LED.
2. Background
Digital dimming is a very popular way to achieve a wide range in average light emitting diode (LED) current. Digital dimming provides greater accuracy than analog dimming in low duty cycles. However, digital dimming has the drawback of the settling time of the current sink or source. In order to achieve very high accuracy in digital dimming, the output current needs to settle to the target current value very quickly. This is especially true where high pulse width modulation (PWM) frequencies, which are outside the audio band, are used and low duty cycles are needed. PWM is an efficient way to provide intermediate amounts of electrical power and is a commonly used technique for controlling power to electrical devices.
Previous solutions to the problem of settling time in digital dimming have turned on and off the gate of the output current source. The output current source has large capacitance due to the large current, such as, 20 mA, and low head room, typically below 250 mV. The turn on settling time is limited by the operational amplifier output stage driving capacity and bandwidth. The turn on time is often hundreds of a nanosecond.
There is a need in the art for drain switching with a replication loop to achieve fast turn on time of an LED drive and provide accurate PWM dimming over temperature at high PWM frequency and a low duty cycle.